Exclusive: details on Hyundai’s new battery thermal management design

Hyundai’s new 2019 Kona Electric, with its 64 kWh battery and an EPA-rated 258 miles of range, has gotten many positive initial reviews but until now we haven’t known much about some important aspects of its internal powertrain design.

We knew the basic size, shape, and layout of the pack. We were pretty sure the battery cells inside the Kona pack were liquid-cooled unlike all previous plug-in cars from Hyundai and its sister company Kia. Those earlier designs used a fan to actively blow cabin air through the inside of their packs.

However, many questions remained unanswered.

The electric motor and related power electronics usually need liquid cooling but would they now be on their own separate cooling loop or would there be some way to exchange heat between them and the battery’s new coolant loop?

And just how does heating the battery work in colder weather? And what do the battery cells even look like? For the Kona Electric we had some tentative hints in our “first drive” review but it was still a bit of a mystery.

The Kona Electric’s battery pack sits under the floor of the vehicle, but what’s inside?

We already know the basic answers to these questions for the Chevrolet Bolt EV, the Jaguar I-PACE, and even the soon arriving Audi e-tron. Now, Hyundai has revealed these details to Electric Revs for their new generation of all-electric cars.

First off, what does the battery pack look like with the lid popped off? Hyundai showed a cutaway version of a Kona Electric prototype at the car’s official introduction at the Geneva Auto Show earlier this year in March.

These two images above show a topless battery pack with the left (driver’s) side of the seat and floor also removed along with part of the rear seat. Each of the dark gray strips seen under the floor is an individual lithium ion pouch cell mounted in a carrier frame (as illustrated below) and collectively packaged into a battery module.

Each cell is held in a frame. The electrical tab on the top and bottom of one of the frames shows how three cells are wired together in parallel to form a cell group.

The cells in the Kona Electric are made by LG and use a cathode chemistry known as NMC 622 which stands for a ratio of 60 percent nickel, 20 percent manganese, and 20 percent cobalt. The cells in the Kia Niro EV are similar but are made by SK Innovation.

This image shows the type of LG pouch cell used in the Chevrolet Bolt EV. It is about 4 inches high, 13 inches long, and 0.6 inches thick and weighs about 1.8 pounds. The Hyundai Kona Electric, Kia Niro EV, Jaguar I-PACE, and Audi e-tron use cells with a roughly similar shape, size, and energy capacity.

According to Jerome Gregeois, a senior manager at the Hyundai Kia America Technical Center, the full 64 kWh pack consists of five modules. Three are under the main cabin floor and two stacked modules are under the rear seats. Each module consists of cell groups as seen in the illustration below (the second stacked module below the rear seats is not shown).

The 294 cells in the pack are wired together three at a time into 98 cell groups which are shown as alternating blue and brown bands in the image. Each of the three floor modules has 20 cell groups. The two stacked modules each contain 19 cell groups.

Here’s an “exploded” illustration of the pack.

The battery modules sit above cooling plates that channel the same type of water and glycol mix that is used for cooling conventional gasoline engines except the heat emitted from batteries is not normally as intense.

Inside the 64 kWh pack there are three coolant sub-loops running through the five modules. One sub-loop runs through the driver-side floor module and one of the stacked modules. A second sub-loop runs though the passenger-side floor module and the other stacked module. The third sub-loop cools only the middle floor module.

What about when the pack needs to be warmed in winter conditions?

When Hyundai initially briefed the media on the US version of the Kona Electric there was some ambiguity about whether the car would have a dedicated battery heater. The company engineers have designed the Kona as a global vehicle with a menu of engineering choices that can be tailored to each marketing region to optimize for price and performance in different climate conditions.

At least for the 2019 model year, all Kona Electric’s sold in the US will come without a dedicated battery heater while all Canadian versions will include one. Similarly, all US deliveries will come with 5.5 kW PTC direct resistive cabin air heating alone while all Canadian deliveries will include a heat pump (reversible A/C system) to more efficiently assist cabin heating.

These choices implicitly assume that most, or at least a very large fraction, of the US deliveries will go to areas of California that rarely experience severe cold weather. Skipping the heat pump and dedicated battery heater saves money and helps lower the consumer price of the vehicle.

Despite the lack of a dedicated battery heater, the US version of the Kona does have the ability to scavenge heat from the electric motor and power electronics in addition to the heat dissipated by the battery itself to help keep the battery warm when operating in colder conditions.

There is one overall thermal management loop with computer-controlled valves that allow a battery pack sub-loop to either run separately or join together with the coolant that runs through the motor, motor inverter and other power electronics, and the on-board (AC) battery charger.

When a dedicated 2 kW battery heater is available (as in the Canadian version), it is used primarily at sub-zero temperatures (0C or 32F) or when the driver enables an optional “Winter Mode”. The battery heater, if present, is located outside the battery pack and warms the liquid “coolant” just before it enters into the pack.

The winter mode uses extra energy to warm the battery pack to allow for full regenerative braking and quicker fast DC charging. Colder pack temperatures force the battery management system to restrict the amount of power that can recharge the battery in order to avoid damaging the carbon graphite anode. This is an issue common to most lithium ion batteries. Cold temperatures are not much of an issue for power coming out of the battery except under rare and extreme conditions like down near -40 degrees.

This graph is based on the cold temperature charging behavior of the Chevrolet Bolt EV which is believed to use similar LG cells and battery management as the Kona Electric. The data shows that the battery can only charge at half the current (60A) at 9C (48F) than it can at 20C (68F). Source: IVI

The chart below, supplied by Hyundai, shows the different operating modes of the dynamically configurable thermal management loops.

Three coolant loop modes

The three modes (Heating, LTR or Low Temperature Radiator, and Chiller) correspond to the three different computer-controlled valve settings and coolant flow diagrams.

During much of the year in mild climate conditions the thermal system typically starts up in LTR mode (labeled “Cool Condition” above) which circulates coolant through a single interconnected loop to warm the battery up to its optimal operating temperature when cold and to maintain that temperature with the help of a radiator and fan.

The three-way valves switch to Chiller Mode (labeled “Hot Condition” above) when the battery starts to get too warm. Hyundai hasn’t said what the exact parameters are. The coolant flows through a “chiller” which exchanges heat with the vehicle’s air conditioning refrigerant loop.

In the Chevrolet Bolt EV, the A/C system begins helping to chill the battery coolant when it reaches much above 32C (90F). But the Bolt has a dedicated coolant loop just for the battery and no valves to allow the exchanging of heat with the motor and power electronics loop.

When temperatures are really cold, the dedicated battery heater kicks in (if present) even if “Winter Mode” isn’t enabled. Like a hot battery in Chiller Mode, the battery coolant sub-loop circulates independently because the battery heater is only needed to warm the battery and not the rest of the components.

This thermal management strategy is somewhat similar to that used by Tesla, and the startup automaker Rivian among others.

The recently introduced Tesla Model 3 can flexibly connect its coolant loops and does not have a dedicated battery heater but it does reportedly have the ability to generate excess heat from its motor and power inverter by deliberately operating inefficiently which is then used in place of a dedicated heater.

Rivian also has a similar flexibly configurable thermal loop that includes a dedicated battery heater.

Owner experiences will demonstrate over the next year how well Hyundai’s design and configuration choices perform in the real world.

Hyundai and Kia are using this new overall battery pack and thermal management design in the 2019 Kia Niro EV and the 2020 Kia Soul EV as well as in the 2019 Hyundai Kona Electric.

The companies also market a smaller 39.2 kWh version of the battery pack outside of the US and Canada. That smaller pack reportedly skips the two modules under the rear seat and reorganizes the three modules under the floor into 90 cell groups using pairs of the same cells used in the 64 kWh version of the pack. This smaller pack continues to use the same liquid-cooled thermal management design as the larger 64 kWh pack.

According to Kia, the North American version of the 2019 Niro EV will come standard with a heat pump to more efficiently assist with cabin heating and the dedicated battery heater will be an optional feature. In the 2020 Soul EV, both the heat pump and battery heater are listed as optional features.

29 replies

Sounds like they have it right this time. I’d be glad to test it for them in our Phoenix Arizona area. We have seen 20 KIA SOUL EV that failed. I even opened a National Highway Safety issue when the controller also over heated and failed twice in the HEAT right during traffic for my wife.
We have also had 2 Nissan LEAF that failed and hundreds of friends with failed LEAF in the HEAT. Desert HEAT testing is the best way to check for this problem. PS My Chevy SPARK EV and Tesla model, 3 have been great in the HEAT with no problems.

Great post, Jeff. The Kona modules look like 1st-cousins to the Bolt EV’s modules. Similar module architecture. The cases are almost identical. Same cell-group arrangement, except 98S3P instead of 96S3P, which could explain why the Kona pack is rated at 64 kWh vs the Bolt’s 60 kWh. Same amp-hours, but a little higher pack voltage. Very similar cell carrier frames that appear to conduct heat from the end tabs and cell top-bottom to the chilled bottom plates. I wonder if there are any between-cell passive conductive “fins”?

Hyundai has more bottom-plate cooling loops with fewer cells per sub-loop than the Bolt, which allows for higher total pack glycol flow and probably improves module cooling, allowing the higher fast charge rates.

The thermal loop schematic diagrams don’t show how the Hyundai switches the AC system to heat pump mode and if it is using the AC condenser coil as the evaporator when in heat pump mode, That would be very interesting to know.

Kia has actually released this information back in November. Not sure how final it is, of course. They list “Heat Pump HVAC system” as standard for both US trims and “Battery Heating System” as optional. There is also standard “High Voltage Positive Temperature Coefficient (PTC) Heater”, but I’m not sure what it is. One can find this information on the official kiamedia site (go Niro –> Niro EV –> Features & Options).

I was actually wondering about the importance of these features. Thanks a lot for a very illuminating write-up!

Positive temperature coefficient means that the hotter the element gets the higher it’s electrical resistance becomes, thereby lowering the allowed current to pass through it. Therefore it is self regulating and not subject to thermal runaway.

Somewhere I read that Hyundai also has added SK as a supplier of batteries because already produced cars where standing and waiting on batteries. It might not be optimal to have 2 different suppliers of batteries to the same car.

There has been the first reports from Norway that it is difficult to get the battery warm, and keep it warm. Even after long trips you can’t charge at max power from quick chargers. Both Hyundai, Kia and i-pace suffers the same problem.
Post on german forum tells that the manual setting for Winter mode has been removed from the cars that has just been delivered (newest FW). Indication is that at temp below 4C the car automatically heats the batteri now.

Hi, Do you know why Kia & Hyundai don’t have the same cell’s supplier ? Aren’t they in the same group ? I saw on some website that both are LG Chem cell’s. Could you tell me where have you seen that SK are the cell’s supplier of Kia please ?

The information about SK Innovation being the primary cell supplier for the Kia Niro EV comes from Jerome Gregeois who is mentioned in the article. He works as a senior powertrain manager for a joint Hyundai and Kia technical center in California and was providing information to me on behalf of the company.

The fact that SK is providing the cells for the Niro had been widely reported already although I’m not surprised that there may be some articles that assume LG provides the cells for both Niro and Kona.

As far as I can tell, the reason for using both SK and LG for what is otherwise essentially the same battery pack design is due to battery supply constraints and competition.

By having both companies supply equivalent cells it protects Hyundai Kia in case SK or LG run into production problems.

It also enhances pricing competition to help drive down battery costs quicker. LG and SK are both now selling as many battery cells as they can make so it also allows Hyundai Kia to get enough supply and gives them established business connections to help expand future supply contracts with whichever company can grow production faster or produce higher quality cells with fewer cell failures. Hyundai Kia can play SK and KG against each other as competing suppliers.

Great article. Is there a difference in the quality of LG and SK batteries? On a lot of EV forums, LG has a reputation for making quality batteries with very little degradation (AESC, not so much). But what about SK? Should we expect any differences in potential battery degradation depending on whether the battery comes from SK or LG? Thanks.

Is there any info available regarding the optional 2 kw battery heater? That is very small, and I can’t find any other info on the internet that mentions this heater. Do you have a reference? The only other car I know of with such a small battery heater was the Spark EV, but I also can’t find any details about that heater. No idea who is supplying these tiny coolant heaters and what they look like!

I believe the Chevrolet Volt and Bolt EV also use a 2 kW battery heater. I don’t know who makes the Spark EV and Volt battery heaters, but the Bolt EV Heater is made by LG. By contrast, I think the Tesla Model S has a 6 kW battery heater.

The Bolt EV and Hyundai Kona battery heaters look superficially similar in illustrations that I have seen but I have not carefully compared them in person or in closeup photos. In both cars, the battery heater is is just outside the battery and heats the fluid right before it enters the battery pack.

You can see what the Bolt EV battery heater looks like by skipping to 30 minutes and 50 seconds into this YouTube video:

Great Article – thanks.
I can confirm that there are 98 cell triplets using BMS data from the battery itself.
At 100% SOC on a Kia Niro EV I am seeing a cell voltage of 4.14V and a battery voltage of 405.5V
Hence or the Kia Niro EV 405.5 / 4.14 = 98

I did not specifically discuss this question with Hyundai since I was focused on documenting the details on the 64 kWh version sold in North America. However, I have seen or heard nothing to indicate that the 39 kWh version is different. So, I am reasonably confident that it is also liquid-cooled.

This is super interesting and thank you for the work you put into it. For a layman whats the main take away of this? I live in virgina and it can get around 22F. Does the fact it doesnt have a dedicated heating element or a heat pump matter? Does this mean it will charge slower during cold weather? Will it will lose range faster during cold temperatures compared to a heat pump system?

However, heat pumps tend to work best above 32F or at “warmer cold” temperatures where the heat in the outside sir can be readily harvested. If the outside air gets much colder then heat pumps have little advantage over resistive heating.

I don’t think that many people got their Kona’s in the US during truly cold weather this year so we may have to wait until next year to know from practical experience how much of a problem it is.

If I lived in a cold weather state or region I would probably get a Kia Niro EV or 2020 Kia Soul EV with the battery heater option if I was planning to do much fast DC charging in winter. If you are just driving locally and charging at night on 240 AC then you are probably ok. Again, the heat pump option is probably useful when traveling longer distances in moderately cold conditions but in really cold areas or when only driving around in town and charging overnight at home (or at work during the day) then resistive heating is probably okay.

Unfortunately, while these are options on the Kia models the battery heater and heat pump are just not available on the Hyundai Kona (at least in the 2019 Model year).